1. Field of the Invention
This invention relates to the field of high power rectifiers.
2. Description of the Related Art
Semiconductor devices are increasingly required to accommodate high currents and/or high voltages without failing. For example, a variable speed pulse-width modulated (PWM) motor control circuit typically employs a number of transistors as switches, each of which has a flyback rectifier connected across it; the switches are closed in sequence to provide variable frequency AC power to a motor. The rectifier in this type of application is required to conduct a large current when forward-biased, and to block a high voltage when reverse-biased. To maximize the efficiency of the control circuit, the flyback rectifier ideally has a low forward voltage drop V.sub.FD. The rectifier should also have a small stored charge Q.sub..PI., to reduce switching loss and to increase switching speed, and a "soft" recovery with a small peak reverse current I.sub.RP, to reduce the stress on the associated switching devices.
A number of power rectifier devices have been used to provide the high current and reverse blocking characteristics needed for such a high power application. One such device, the P-i-N rectifier, is shown in FIG. 1. An - drift layer 10 is between an N+ layer 12 and a P+ layer 14 (X+ denotes a carrier concentration of at least 1.times.10.sup.18 /cm.sup.3, X- denotes a carrier concentration of less than 5.times.10.sup.16 /cm.sup.3). Metal on the P+ and N+ layers provide the rectifier's anode 16 and cathode 18, respectively.
When forward-biased, P+ region 14 injects large numbers of minority carriers into drift region 10, greatly lowering the resistance of the drift region and allowing the rectifier to carry a high current density. The P-i-N rectifier's drift region 10 is usually thick, resulting in a high "blocking voltage"; i.e., the reverse voltage which the rectifier can accommodate without breaking down. These characteristics make the P-i-N rectifier useful for high power applications.
The P-i-N rectifier has several drawbacks, however. As described in J. Baliga, Power Semiconductor Devices, PWS Publishing Co. (1996) at p. 153, the P-i-N rectifier suffers from a "forward voltage overshoot" phenomenon, in which its V.sub.FD at turn-on is higher than it is under steady-state conditions. This can be a serious problem in power circuit because the higher V.sub.FD may appear across the emitter-base junction of a bipolar transistor used as an active element and exceed its breakdown voltage.
Another drawback of the P-i-N rectifier is its poor reverse recovery characteristic--as described in Baliga (ibid.) at p. 154. Reverse recovery occurs when the rectifier is switched from its on-state to its reverse blocking state. To undergo this transition, the minority carrier charge stored in the drift region during forward conduction must be removed, which requires the injected minority carriers to recombine with majority carriers. During recombination, some reverse current flows through the device before eventually decaying to zero Because so many holes are injected into the drift region during forward conduction, recombination proceeds slowly in a P-i-N rectifier and thereby produces a poor reverse recovery characteristic with a large I.sub.RP and large voltage overshoot. This poor reverse recovery characteristic adds a considerable amount of stress to the power switches the rectifier is typically connected across, and requires the rectifier to dissipate a significant amount of power when transitioning to a reverse blocking state.